Fix documentation example

The original example was broken in a few ways:

1. The 2 service containers would exit immediately, so attempting to link to them from the build container would fail.
2. The script job just calls make in the runner repo, which ultimately requires Go, but the job was run in a ruby container :-/
3. Actually running make in the runner repo does A LOT of stuff; way too much to be a practical example (and some of it would probably fail if run from within a docker container since it would require DinD).

The example now actually works!

Fixes #431324 (closed)